Doc: note that statement-level view triggers require an INSTEAD OF trigger.

 Doc: note that statement-level view triggers require an INSTEAD OF trigger.
 
 If a view lacks an INSTEAD OF trigger, DML on it can only work by rewriting
 the command into a command on the underlying base table(s).  Then we will
 fire triggers attached to those table(s), not those for the view.  This
 seems appropriate from a consistency standpoint, but nowhere was the
 behavior explicitly documented, so let's do that.
 
 There was some discussion of throwing an error or warning if a statement
 trigger is created on a view without creating a row INSTEAD OF trigger.
 But a simple implementation of that would result in dump/restore ordering
 hazards.  Given that it's been like this all along, and we hadn't heard
 a complaint till now, a documentation improvement seems sufficient.
 
 Per bug #15106 from Pu Qun.  Back-patch to all supported branches.
 
 Discussion: https://postgr.es/m/152083391168.1215.16892140713507052796@wrigleys.postgresql.org